This invention relates to a method of forming an iron oxide film of, for example, .alpha.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4, which is necessary to form a magnetic recording medium having high packing or recording density for use in a magnetic disk. This invention relates also to a sputtering device for use in carrying out the method.
In order to achieve high packing density in the magnetic disk, it is preferable that a magnetic recording medium has a thin thickness. As such a magnetic recording medium, a recent attention is directed towards a thin film of .gamma.-Fe.sub.2 O.sub.3. Two conventional methods of forming a .gamma.-Fe.sub.2 O.sub.3 film will be described in the following.
In a first method of forming the thin film of .gamma.-Fe.sub.2 O.sub.3, an .gamma.-Fe.sub.2 O.sub.3 film is formed by reactive sputtering at first. Thereafter, the .gamma.-Fe.sub.2 O.sub.3 film is transformed into an Fe.sub.3 O.sub.4 film by first heat treatment and subsequently into the .gamma.-Fe.sub.2 O.sub.3 by second heat treatment. The second heat treatment may be called a gamma transformation process, namely, a .gamma.-izing process.
In a second method of forming the thin film of .gamma.-Fe.sub.2 O.sub.3, an Fe.sub.3 O.sub.4 film is directly deposited by reactive sputtering and is thereafter transformed into the .gamma.-Fe.sub.2 O.sub.3 film by the gamma transformation process.
The second method has a tendency that a phase of iron or .alpha.-Fe.sub.2 O.sub.3 is mixed with the thin film of Fe.sub.3 O.sub.4 during the reactive sputtering. This disables to effectively carry out the gamma transformation process. In this view, it is necessary to stably form the film consisting of a single phase of Fe.sub.3 O.sub.4.
It is important to establish a method of stably forming the thin film of .alpha.-Fe.sub.2 O.sub.3 and the thin film of Fe.sub.3 O.sub.4 in order to achieve mass production of the magnetic recording medium and to thereby manufacture the magnetic recording medium at a low cost.
In a conventional method of forming an iron oxide film of .alpha.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4 on a substrate by reactive sputtering, the substrate is located in a space in face to face relation to a target disposed in the space. The target consists essentially of iron. An introducing gas which is introduced into the space consists either essentially of oxygen or a combination of argon and oxygen. An electric field is produced between the target and the substrate to generate a glow discharge in the space. An electric voltage source is connected between the target and the substrate to produce the electric field. Sputtering of the target is carried out by the aid of the glow discharge. As a result, the iron oxide film is formed on the substrate.
In the conventional method, reactive sputtering is carried out by controlling a voltage which is applied to the target by the electric voltage source, a pressure of the space, and a duration of sputtering so that the voltage, the pressure, and the duration are adjusted to respective preselected values. The pressure must generally be measured far from a region where the glow discharge is generated. It is therefore difficult to measure a true partial pressure of oxygen in the region. In addition, the most suitable condition of the partial pressure of oxygen complicatedly varies by the degree of reaction of a surface of the target and a rate of forming the iron oxide film. On account of the above-mentioned reasons, it is difficult to control the state of the glow discharge at a constant state. It is therefore impossible to stably form the iron oxide film of .alpha.-Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4.
In each of Japanese Patent Publications Nos. 32716/1979, 14058/1980, and 14059/1980, an improved method for stably forming an iron oxide film of Fe.sub.3 O.sub.4 is disclosed wherein sputtering is carried out by accurately controlling the voltage applied to the target. Accurate control of the voltage is carried out so as to maintain a preferred glow discharge. As will later be described with reference to one of several figures of the accompanying drawing, the improved method needs a shutter which is disposed between the target and the substrate. The shutter acts to restrict formation of the iron oxide film to a part of a surface of the substrate. The substrate is rotated so that the iron oxide film can be formed on a whole surface of the substrate. However, use of the shutter inevitably makes the reactive sputtering long so as to obtain a predetermined thickness of the iron oxide film. In addition, such a long sputtering results in an increase of useless consumption of the target and makes it difficult to form the iron oxide film at a low cost.